[NotLanded] Implement the fetching algorithm of the Web Manifest.

Source/core/core.gypi

 {
     'includes': [
       'core_generated.gypi',
     ],
     'variables': {
         # Files for which bindings (.cpp and .h files) will be generated
         'core_idl_files': [
             'animation/Animation.idl',
             'animation/AnimationEffect.idl',
             'animation/AnimationPlayer.idl',
@@ skipping 1366 matching lines @@
             'loader/FrameLoadRequest.h',
             'loader/FrameLoader.cpp',
             'loader/FrameLoaderStateMachine.cpp',
             'loader/HistoryItem.cpp',
             'loader/HistoryItem.h',
             'loader/ImageLoader.cpp',
             'loader/ImageLoader.h',
             'loader/LinkLoader.cpp',
             'loader/LinkLoader.h',
             'loader/LinkLoaderClient.h',
+            'loader/ManifestLoader.cpp',
+            'loader/ManifestLoader.h',
             'loader/MixedContentChecker.cpp',
             'loader/MixedContentChecker.h',
             'loader/NavigationAction.cpp',
             'loader/NavigationAction.h',
             'loader/NavigationPolicy.cpp',
             'loader/NavigationPolicy.h',
             'loader/NavigationScheduler.cpp',
             'loader/PingLoader.cpp',
             'loader/PingLoader.h',
             'loader/PrerenderHandle.cpp',
@@ skipping 2105 matching lines @@
             'rendering/shapes/BoxShapeTest.cpp',
             'rendering/style/OutlineValueTest.cpp',
             'testing/PrivateScriptTestTest.cpp',
             'streams/ReadableStreamTest.cpp',
             'testing/UnitTestHelpers.cpp',
             'testing/UnitTestHelpers.h',
             'xml/parser/SharedBufferReaderTest.cpp',
         ],
     }
 }